The field of electrosurgery includes a number of loosely related surgical techniques which have in common the application of electrical energy to modify the structure or integrity of patient tissue. Electrosurgical procedures usually operate through the application of very high frequency currents to cut or ablate tissue structures, where the operation can be monopolar or bipolar. Monopolar techniques rely on a separate electrode for the return of RF current, that is placed away from the surgical site on the body of the patient, and where the surgical device defines only a single electrode pole that provides the surgical effect. Bipolar devices comprise both electrodes for the application of current between their surfaces.
Electrosurgical procedures and techniques are particularly advantageous since they generally reduce patient bleeding and trauma associated with cutting operations. Additionally, electrosurgical ablation procedures, where tissue surfaces and volume may be reshaped, cannot be duplicated through other treatment modalities.
Present electrosurgical techniques used for tissue ablation suffer from an inability to control the depth of necrosis in the tissue being treated. Most electrosurgical devices rely on creation of an electric arc between the treating electrode and the tissue being cut or ablated to cause the desired localized heating. Such arcs, however, often create very high temperatures causing a depth of necrosis greater than 500 μm, frequently greater than 800 μm, and sometimes as great as 1700 μm. The inability to control such depth of necrosis is a significant disadvantage in using electrosurgical techniques for tissue ablation, particularly in arthroscopic procedures for ablating and/or reshaping fibrocartilage, articular cartilage, meniscal tissue, and the like.
Generally, radiofrequency (RF) energy is extensively used during arthroscopic procedures because it provides efficient tissue resection and coagulation and relatively easy access to the target tissues through a portal or cannula. However, a typical phenomenon associated with the use of RF during these procedures is that the currents used to induce the surgical effect can result in heating of electrically conductive fluid used during the procedure to provide for the ablation and/or to irrigate the treatment site. If the temperature of this fluid were allowed to increase above a threshold temperature value, the heated fluid could result in undesired necrosis or damage to surrounding neuromuscular and/or soft tissue structures.
Previous attempts to mitigate these damaging effects have included either limiting the power output of the RF generator or to include a suction lumen on the distal tip of the electrosurgical device to continuously remove the affected fluid from the surgical site and thereby reduce the overall temperature. These solutions may be effective but are limited and they do not allow for direct feedback based upon the actual temperature of the fluid within the joint space. Furthermore, limiting the power output of the generator reduces the rate of the surgical effect, which is often unacceptable from a clinical perspective. The incorporation of a suction lumen to allow heated fluid to be removed also reduces the performance of the electrosurgical device.
There have been numerous RF based systems introduced into the market that make use of a temperature sensor (e.g., a thermocouple) in order to monitor the temperature of tissue at or near the electrode.
However, the temperature sensors are susceptible to electrical noise. Electrical noise may arise from a number of sources including, for example, (1) high frequency noise present on the electrical circuit used to measure the small voltages induced by the temperature sensor, namely, a thermocouple, or (2) resistive heating of the thermocouple junction arising from the delivery of the ablative energy to the tissue.
Filtering the measured signal to reject the high frequency components can generally remove the high frequency noise described above. However, the error arising from the resistive heating of the thermocouple junction is a physical phenomena that cannot be mitigated by filtering. An improved system and method to accurately monitor the temperature of the fluid is still desired.